This invention relates generally to vesicles and more particularly to polymerizable, lipid vesicles and the method of administering therapeutic agents therewith.
Used herein, "lipid vesicle" and "liposome" refer to a hollow, spherical like structure having an interior which can encapsulate aqueous solvents and their solutes; and having a shell or membrane composed of lipid layers. "Lipid microsphere" refers to a solid lipid spherical like structure such that any substance dissolved in the microsphere must generally be lipid soluble and, hence, hydrophobic or amphiphilic.
Typical methods of treating disease involve intravascular injections of fairly toxic therapeutic agents. To maximize the efficacy at the desired location, the blood plasma concentration of the therapeutic agent must be kept at a high level throughout the body. This approach produces undesirable side effects since the therapeutic agent acts upon many in vivo systems. Encapsulation of the therapeutic agent into lipid microspheres and targeting the microspheres to a specific release site within the body have been suggested to avoid many undesirable side effects caused by high blood plasma concentrations. The therapeutic agent is encapsulated into microspheres containing ferromagnetic embedded particles. The microspheres are injected into the body upstream of the target site, allowed to flow in the blood stream to the target site, and immobilized by an external magnetic field. The therapeutic agent leaks into the target site or is released by in vivo enzymatic action. This produces a high concentration of the therapeutic agent at the target site while keeping the overall concentration in the blood plasma low thereby minimizing undesirable side effects on the rest of the body.
There are, however, problems and limitations with this technique: only fat soluble therapeutic agents can be easily encapsulated in lipid microspheres and, after injection, the microspheres may be physically or chemically degraded before they reach the target site. Additionally, there is no effective means to control the rate of release of the therapeutic agent. Current methods rely on leakage of the therapeutic agent from the microsphere or on in vivo enzymatic activity that degrades the microsphere at an indeterminate rate.
U.S. Pat. No. 4,247,406 discloses a magnetically localizable, biodegradable microspheres formed from an amino acid polymer matrix embedded with magnetic particles. The microspheres were formed from matrix materials, such as albumin, which could be attacked in vivo by proteolytic enzymes thus releasing the microsphere's contents. To obtain slower release rates, enzymatic activity was inhibited by crosslinking the matrix using known hardening agents. No method of increasing the life time or stability of the vesicles was disclosed. The release rate for the microspheres' contents was dependent on the degree of crosslinking and the inherent proteolytic enzyme activity in vivo.
U.S. Pat. No. 4,345,588 discloses a method for immobilizing therapeutic agent carrying microsphere in a capillary bed using an 8000 gauss magnetic field following injection of the microspheres into an upstream artery and subsequent migration of the vesicles to the desired capillary bed. No method for releasing the therapeutic agent is disclosed. The microsphere used were formed from nonpolymerizable lipids using the techniques known in prior art. The therapeutic agent is carried by amino acid polymers which are hardened by denaturation of the protein or by hardening with formaldehyde.
U.S. Pat. No. 3,474,779 describes a method for administering therapeutic agents in which magnetic microspheres are intravascularly administered so that they pass into a capillary bed where they are caught by an applied magnetic field, and magnetically retained in the capillary bed until the therapeutic agent contained in the microsphere is released.
Microcapsules containing magnetic particles are disclosed in U.S. Pat. No. 2,971,916. The microcapsules of up to 150 micrometers in diameter are formed by coacervation, the capsules having walls of hardened organic colloid material enclosing an oily liquid containing a dispersion of magnetic powder.
U.S. Pat. No. 3,663,687 teaches the use of biodegradable microspheres for intravascular administration of therapeutic agents. The microspheres are dimensioned so that they will lodge in the capillaries where they can be degraded by enzymatic action thus releasing the therapeutic agent. The patent further teaches that the rate of release of the therepeutic agent can be varied by cross-linking the protein material forming the microspheres.
Lipids have been used to encapsulate therapeutic agents in an effort to selectively administer them to target sites. Rahman, Proc. Soc. Exp. Biol. Med., 146, 1173 (1974), studied the effect of liposome encapsulated antinomycin D on tumors in mice and concluded that mean survival times for those mice treated with encapsulated therapeutic agents increased.
Gregoriades, Biomedical and Biophysical Research Communications, 65:537 (1975), studied the possibility of holding liposomes to targeted cells using liposomes containing anti-tumor drugs.
Sozka et al., Am. Rev. Biophys. Bioeng., 9:467-508 (1980), reviews the known methods for producing and characterizing lipid vesicles. Dispersion, sonication, detergent solubilization and dialysis, solvent injection, reverse phase evaporation, extrusion, fusion and freeze/thaw techniques are typical methods used to produce lipid vesicles. A vesicle with desired size and stability characteristics can be produced using one or more of the above techniques in combination with various separating procedures such as column chromatography.
Thus, the prior art is lacking a rugged and stable means for transporting therapeutic agents to in vivo targeted sites. The vehicles used are subject to premature enzymatic attack and physical degradation. Additionally, current methods rely on timely enzymatic activity or leakage to release the encapsulated therapeutic agent at the targeted site. Neither method permits a controlled release of the therapeutic agent at the site.